Acoustic echo is a major problem in communication devices designed in such a way that a near end microphone is exposed to a loudspeaker driven by a far end signal. When the near end microphone picks up the far end loudspeaker signal, the communication device retransmits the signal to its original source. This causes far end users to hear a delayed version of their speech referred to as acoustic echo.
One way of avoiding acoustic echo is to operate the communication device in half duplex mode. In half duplex mode, communication signals only proceed in one direction at a time. This prevents acoustic echo, but it causes degradation in an interactive conversation.
Another way of preventing acoustic echo is to use an echo canceller. Echo cancellers allow communication devices to operate in full duplex mode which provides the maximum interactive conversation.
In general, an echo canceller is an adaptive filter that models the electroacoustical response of a near end microphone when a far end loudspeaker signal is received as input. The modeled response of the near end microphone is then subtracted from an outgoing communication signal to yield an echo reduced communication signal. During operation of the echo canceller, the coefficients of the adaptive filter are adapted so the filter can converge on an accurate estimate of the echo signal.
One example of an echo canceller is described in U.S. Pat. No. 5,305,307 entitled "Adaptive Acoustic Echo Canceller Having Means for Reducing or Eliminating Echo in a Plurality of Signal Bandwidths" issued to Chu. Another example of an echo canceller is described in U.S. Pat. No. 5,526,426 entitled "System and Method for an Efficiently Constrained Frequency Domain Adaptive Filter" issued to McLaughlin.
Despite the numerous advances that have been made in echo cancellers, conventional devices still have limitations. For example, conventional echo cancellers exhibit less than optimal echo reduction after converging to their peak performance levels.